The U.S. military has gone cave-hunting, looking for holes
that may be home to bin Laden and his al-Qaeda terrorist network.

With Taliban troops on the run, military planners have
turned their efforts to rooting out al-Qaeda terrorists from their hiding places.
In Afghanistan, the cave hunting has begun, with "bunker-buster" bombs targeting
mountain hideouts and special operations troops investigating abandoned tunnels.
Defense Secretary Donald Rumsfeld has warned that the hunt for Osama bin Laden
and his followers may be a "long haul." Scientists familiar with cave detection,
whether geologists looking for oil, archaeologists searching for lost cities
or engineers uncovering a misplaced utility line, agree that the long haul is
often what it takes to find something hidden underground.

"While it's not always hard to find something, even when it's
easy it's time-consuming," says geophysicist Alan Witten of the University of
Oklahoma in Norman.

Luckily, he and other experts say a bevy of techniques
exist for snooping out tunnels and caves, some experimental, some tried-and-true,
many of them familiar to military planners.

So far, pilots have been using heat detectors, which pick
up the infrared energy emitted both by people and cave entrances.

But they may have to look hard. Afghanistan's caves have
served as hideouts for guerrilla forces for centuries. In the mountains, extensive
limestone formations hide long, twisting subterranean passages. On the southern
plains, where Taliban forces now are concentrated, ancient karez irrigation
tunnels stretch for miles carrying water to farms. The tunnels, punctured by
wells about every 300 feet, have long served as hiding places for men and arms.

Finding underground passages will be a substantial challenge,
geophysicists say, even though the area they're searching is limited (Afghanistan
is a little smaller than Texas). There are two types of techniques for finding
underground spaces, experts say: ground-based or airborne. Ground-based techniques
are much more effective because they place the sensing device closer to the
target. But they're also more time-consuming and vulnerable to ambush, says
geophysicist Brian Spies of the Australian Nuclear Science Technology Organization.

All techniques require lengthy readings of the earth over
a grid-defined area, using sound waves, radar or other signals to detect voids
underground.

Advances in computers allow scientists to winnow signs
from empty spaces in ways impossible only a decade ago.

Ground techniques include:

 Seismic. Long used by the Army to detect North Korean
tunnels, it involves sending sound waves into the earth and listening for echoes
from caves with sensitive earphones hooked to computers. The method goes deeper
than others but requires expert interpretation and more time.

 Earth resistance tomography. Electrodes placed into
the ground map out the resistance of the earth below. Witten's students have
detected subway tunnels 45 feet deep. U.N. inspectors used this technique in
Iraq to search for weapons caches.

 Ground-penetrating radar. Radio waves are sent into
the earth; echoes from voids are picked up from returning noise. This is commonly
used to detect misplaced utility tunnels as deep as 20 feet.

 Microgravity. Using a device called a gravimeter,
scientists can detect subtle changes in gravity's pull above caves as far down
as about 100 feet. NASA funds the development of such devices for its science
missions. Scientists measure the pull of gravity a number of ways, most often
with lasers that time the fall of an object over a precise distance. Gravity
will be lower, perhaps by a factor of one-millionth, over a cave.

Much of the technology involves heat detectors, or "thermal
imagers," that see infrared energy. Hotter objects, including cave openings
and tunnel air vents, show up on viewing devices (such as Army "night vision"
goggles) as bright spots that stand out from their surroundings.

Other airborne techniques:

 Electromagnetics. A low-flying plane equipped with
an oversized antenna measures the electrical resistance of the ground, stuffing
the data into a computer as the plane flies back and forth over a series of
grids. Voids appear in the data as areas of high resistance, because air doesn't
conduct electricity as easily as rock does.

 Satellite imagery. From space, geological signs
of tunnels may reveal themselves in depressed features on the ground. But the
signs are easily confused in jumbled terrain.

 Radar. Some companies use ground-penetrating radar
for geological surveys. In a project under development, the Air Force Research
Laboratory in Rome, N.Y., has awarded a $600,000 contract to develop ground-penetrating
radar for tunnel detection that would work from the air. Lab officials will
not discuss the contract winner's proposal.

Typical cave hunters use methods that are not as high tech,
says Nick Crawford, head of the Center for Cave and Karst Studies at Western
Kentucky University in Bowling Green. Karst is the name for cave-riddled regions
such as western Kentucky or Afghanistan.

To find cave entrances, spelunkers might pour harmless
dye into a mountain stream that disappears underground, searching from a helicopter
for where the color emerges farther downhill to lead them to an entrance. Or
they might blow smoke into a cave entrance, watching to see where it emerges,
to find new caves. Another technique is to look for sources of radon, a radioactive
gas released underground, as a giveaway of entrances.

But Witten says the most valuable cave-finding means in
Afghanistan will be people, civilians or former mujahedin, with knowledge of
the caves used for hiding from Soviet troops during the 1980s and subsequent
tunnel construction. Just using human knowledge to narrow the location of a
tunnel can speed things greatly, Witten says.

"Give me an acre to search, and I'll probably find it within
a month. Give me a vault anywhere in the country, and I'll be lucky to find
it in a century."